Strapless Bra's?

I'm a bridesmaid at my friends wedding in July, my dress is lovely but the style does dictate a strapless bra is required. I've not had great experiences of these in the past, finding myself with either 'cone shaped boobs' or everything squashed flat by a bra that's like a bandage! Any recommendations would be great, I'm a 34DD and resigned willing to pay up to about £40.

I have a great one from Bravissimo. Actually I have 2 as I bought another cos I wear that first one so much it's wearing out! It's the Freya Deco. Doesn't fall down at all and doesn't squash boobs either.